1. Field of the Invention
This invention relates to ozone generators and is more particularly concerned with a method and apparatus which allows for the controlled production of ozone gas by the direct application of electrical energy input.
2. Description of the Prior Art
Many authorities believe ozone gas to be a superior oxidizing agent. Devices to manufacture ozone gas have before been inefficient and costly to build thus severely limiting their practical application for the private or commercial generation of ozone gas. Therefore, the oxidative treatment of waste fluids, gases, and solids has heretofore been accomplished with alternate oxidative agents such as various dioxides, peroxides, chlorine, or other halogenated compounds. These compounds are not only dangerous during periods of acute exposure, producing a variety of illness syndromes, but several of these compounds as well as their by-products are known to be potent carcinogens. Some examples of these carcinogenic substances would include chloramines, tri-chloroethane (TCE), and tri-halomethanes (THM). In addition most of these alternative oxidizing agents require the potentially hazardous steps of manufacture, transportation, and storage. Oxidative treatment of substances by ozonation avoids the hazards of transportation and storage of these dangerous compounds as ozone gas may be both made and used on site. Because of its superior oxidizing capability, second in nature only to elemental fluorine, ozone gas probably inactivates the majority of these halogen-based carcinogenic compounds rendering them non-carcinogenic, (Burleson and Chambers, Enviromental Mutagenesis, 4:469-476, 1982).
Ozone is generally formed by the action of oxygen atoms on oxygen molecules. The splitting of the oxygen molecule can be achieved by applying electrical, optical, chemical or thermal energy. As is well known, splitting of the oxygen molecule may be effected by subjecting oxygen to thermal energy. However, this method is inefficient since elevated temperatures used to produce ozone by heating, or with chemical reactions which cause heat production, favor the thermal degradation of ozone immediately as it is produced. Ultraviolet light ozone generators, such as those disclosed in Pincon U.S. Pat. No. 4,124,467, and Beitzel U.S. Pat. No. 4,189,363, produce concentrations of ozone suitable only for minimal decontamination, purification, or oxidative treatment.
Other means of generating ozone have generally involved electrical means, i.e., corona discharge units (using either cylindrical tubes or flat plate generators) and hollow-cathode plasma discharge assemblies. The cathode plasma discharge assembly, such as that disclosed by Orr, Jr. et al U.S. Pat. No. 4,095,115, produces ozone gas by exposing an oxygen enriched medium to a high-energy electron beam causing splitting of the oxygen molecules into singlet oxygen and/or ozone. Devices of this sort produce only a maximum concentration of less than 500 parts per million ozone in air but require extremely high velocities of gaseous injection to achieve even this modest concentration of ozone. Additionally, the overall efficiency of this system is compromised because of the energy requirements of pumps, gauges, power supply inefficiencies, etc.
Other electrical means of generating ozone gas universally depend upon the cold spark of corona discharge in order to split the oxygen molecule. Corona discharge units commonly depend upon the field intensified ionization of an oxygen bearing gas which occurs within an insulating system. The luminous discharge of electricity due to the ionization of the gas within such an insulating system will occur when the field potential gradient of an alternating current exceeds a certain value termed the corona start voltage (CSV). If the strength of the insulating system is not exceeded and the system does not immediately break down in a catastrophic manner, then a non-disruptive electrical discharge will occur and continue until the voltage is reduced to the corona extinction voltage (CEV). Corona extinction voltage is always at a lower potential than the (CSV). This is well illustrated in attached FIGS. 4a and 4b.
The values and relationships of the (CSV) and (CEV) are important in that they define the period of latency; that period of time during each cycle of alternating current application in which no corona discharge is present within the tube to produce ozone gas. The alternating current cycle is bidirectional in that it travels both above and below ground potential. As the wave of electrical potential is passing upward from ground as in FIG. 4a at point TO and at (CEV) point T1, insufficient ionization potential is present within the corona discharge gap to permit corona formation and subsequent ozone production. It is not until the wave exceeds (CSV) at point T2 that corona discharge begins. Ozone producing corona discharge will continue as the wave travels in the direction of point T3, its maximum positive excursion, and continues until the wave potential drops below point T5 the (CEV). The wave continues its fall through ground potential at point T6 and (CEV) at point T7, until reaching the (CSV) (in the opposite polarity) at point T8. Corona discharge is re-established and continues through the maximum negative excursion at point T9 and through (CSV) at point T10 until reaching (CEV) at point T11 traveling toward and reaching ground potential at point T12. FIG. 4a graphically illustrates that there is a significant period of time during each cycle of alternating current application wherein no ozone gas can be produced (illustrated by the broken lines in the electrode potential waveform). In any corona discharge apparatus, this less-than-optimal cycle repeats itself at the frequency of alternating current applied thereto.
On the molecular level, these devices produce ozone by bombardment of the oxygen molecule with high-energy electrons causing a splitting of the diatomic molecule into charged oxygen atoms or singlets. These may randomly recombine with one or more charged oxygen atoms to produce oxygen (02) or ozone (03). They may also, in a random fashion, recombine with diatomic oxygen molecules to produce ozone. Because of the random nature of this recombination of oxygen atoms and oxygen molecules, a metastable equilibrium state will be achieved. Concentrations of up to 5% of ozone in pure anhydrous oxygen can be prepared in this manner. The oxidative treatment of large volumes of fluid, solid and/or gaseous substances will require a proportionately large volume of ozone gas. The capital expenses of purchasing, storing, and utilizing pure oxygen may limit its practical and economic merit for use in ozone gas generation. Air, having an oxygen concentration of approximately 21%, is a reasonable alternative source of oxygen molecules but has the drawbacks of being composed mainly of nitrogen (approximately 78%), and also having variable amounts of water vapor diffused within it. When exposed to high temperatures and high voltages (typically greater than 15,000 Volts), the nitrogen molecules in air will ionize and break into singlets which may combine with oxygen atoms or molecules to form undesired nitrous oxide compounds. This situation can be further complicated by the reaction of hydrogen atoms from water vapor reacting with the nitrous oxide compounds to produce unwanted and toxic nitric acids. Additionally, water vapor detrimentally lowers the spark voltage in the coronal discharge chamber further limiting the efficiency of ozone gas production.
Previously described flat plate corona discharge units, such as those disclosed by Lowther (U.S. Pat. No. 3,996,474) and Erz et. al. (U.S. Pat. No. 4,545,960), have utilized materials of high dielectric constant which are breakable, fragile, expensive to manufacture, generate excessive heat, and require elaborate schemata to remove the heat produced. Electrical energy for ozonation may also be supplied for example by the so-called "Siemens ozonizer" and variations therefrom, which are in essence devices comprised of two telescoping coaxial glass tubes whose outer and inner walls respectively are made electrically conductive, are water cooled, and which are electrically connected to the terminals of an alternating current power supply. Electric discharges take place in the narrow annular chamber between the glass tube walls when an alternating current is applied thereto, a dry system of oxygen or air being passed through this chamber. Multiple variations of this basic theme have been described in the prior art, (Ito U.S. Pat. No. 4,725,412, Hirth U.S. Pat. No. 4,690,803, Sasaki U.S. Pat. No. 4,696,800, and Slipiec et al U.S. Pat. No. 3,967,131).
Apparatus of this nature, while having been much improved in the meantime, are still bulky, cumbersome, difficult and costly to manufacture, poorly durable, and they employ rigid fragile materials in their manufacture and construction. For example, Hirth's ozonizer tube utilizes a rigid dielectric member coated with a glaze of titanium oxide ceramic and therefore achieves a high dielectric constant with an increased spark voltage and increased ozone production, but only at the cost of degradation of a portion of the newly generated ozone due to heat accumulation. This construction is not only expensive but relatively fragile. Any break, crack, or puncture of the rigid dielectric shield member will cause high voltage, disruptive arcing and self destruction of the device thereof. In addition, there must be practical limitations to the physical length of such an ozonizing tube, which therefore limits the surface area and volume of a gas which can be exposed to ozonizing currents at any one point in time.
Any attempt to increase the volume of gas which can be exposed to ozonizing current by simply increasing the annular space in a rigid ozonizer tube will result in reduced ozone production because the electric field density will be proportionately diminished. This phenomena is a well known law of nature (the inverse square law) wherein the strength of field intensity decreases in proportion to the square of the distance from the energy source.
Only a portion of the energy of corona discharge (about 34 kcal. per gram mole) is required for formation of ozone (Handbook of Chemistry and Physics 69th Ed. 1988-89, Library of Congress #13-11056). The remaining energy will dissipate as heat and light. If this excess energy is not rapidly transferred from the system, then the temperature of the effluent gases, electrode, dielectric shield, and housing will rise. This may lead to a more rapid decomposition of a portion of the ozone which, at approximately 100 degrees Centigrade, breaks down almost as soon as it is formed. Prior attempts at increasing ozone production have stressed the utilization of insulators with a high dielectric constant such as glass or ceramics interposed between the electrodes. The prior art has emphasized the importance of using insulators with dielectric constants ranging from 8-12,000 in order to achieve their stated goal. Despite the increased corona spark voltage and the increased production of ozone therefrom, the heat generated by electrical excitations in these materials of high dielectric constant causes loss of ozone through degradation due to excess heat formation. Of even greater importance is the extreme total energy loss produced by dielectric heating per se (Modern Electronics Communications, by Gary Miller, 1978, Library of Congress #77-25881, page 364) and (Buchsbaum's complete Handbook of Practical Electronic Reference Data, 1978, Library of Congress #78-1055, second edition page 551).
Dielectric losses result from heating of the insulating materials between the electrode and counter-electrode when an alternating current is applied thereto. Materials most susceptible to this type of heating are known as "lossy" type dielectrics. A means for predicting losses caused by dielectric heating is easily determined by the product of the dielectric constant and the power factor. The power factor is the ratio of resistance to impedance of the dielectric material.
The production of heat within a dielectric requires energy which must be taken from the power source. Application of an alternating current electric field formed by the alternating potential difference between the two electrodes (electrode and counter-electrode) across an insulating dielectric member will cause distortion of the normal electron spin paths of the atoms comprising the dielectric insulator. The electron paths will be altered because they will be alternately repelled by the negative potential of one electrode and attracted to the positive potential of the counter electrode and vice versa. The structures of the atoms of some materials are harder to distort, i.e., glass, ceramic, polyvinylchloride (PVC), rubber; thus more energy is absorbed from the power source. The electron paths of some atoms are easily altered and require very little energy from the source, i.e. polyethylene, polystyrene, polytetrafluoroethylene (PTFE), and silicone materials. As a general rule, insulators with a low dielectric constant will have more easily altered electron spin paths and hence lower susceptibility to dielectric heating.
Distortion and subsequent heat formation in the dielectric material are directly proportional to the peak voltage applied across the dielectric as well as the frequency of the alternating current source. Even though dielectric losses play a significant role in the deficiencies of the prior art leading to inefficiency in the generation of ozone, of paramount importance is the previously unrecognized significance of maximizing energy transfer to the corona discharge gap.
The multiplicity of constraints of the prior art concerned with the methods and apparatus for the production of ozone gas including the electrical and mechanical inefficiencies, the relatively large size of previous chambers, and the expense of construction, has limited the practical applicability of the heretofore described devices and methods. Both the theoretical and practical improvements of the present invention will become clear as the discussion proceeds.